## calculation of soil weight behind a retaining wall

## calculation of soil weight behind a retaining wall

(OP)

Hi..

I want to calculate the resistance forces against sliding in a retaining wall design, the water table is in the surface, so should I use the total unit weight or the the effective unit weight to calculate the soil weight behind the wall? the total unit weight for the soil is 18, so should I use 18 or (18-10) because of the presence of the water?

I want to calculate the resistance forces against sliding in a retaining wall design, the water table is in the surface, so should I use the total unit weight or the the effective unit weight to calculate the soil weight behind the wall? the total unit weight for the soil is 18, so should I use 18 or (18-10) because of the presence of the water?

## RE: calculation of soil weight behind a retaining wall

However, I would use the total unit weight (~18 kN/m3) for the "resisting" horizontal force (weight of soil on the heel multiplied by the friction coefficient at the base of the wall).

## RE: calculation of soil weight behind a retaining wall

## RE: calculation of soil weight behind a retaining wall

## RE: calculation of soil weight behind a retaining wall

I have used unit weight 24 Kn/m3 for the concrete to calculate the stem and base weights, and a unit weight 18 Kn/m3 for the soil on the heel to calculate its weight. is this alright?

## RE: calculation of soil weight behind a retaining wall

## RE: calculation of soil weight behind a retaining wall

Dry wood won't sink, but is buoyed up by the volume of water displaced. So concrete being of heavier unit weight than water will have some weight still under water. of course I am only talking the reduction in effective unit weight for that part of the concrete below the water surface elevation.

## RE: calculation of soil weight behind a retaining wall

If you don't do provide the drainage, then your retaining wall will be like a dam - check out your text books for flow nets on a dam. Draw a flow net and you will see what OG is saying - the wall will have uplift pressures on the base. These uplift pressures will have to be included in your force/moment diagrams.

## RE: calculation of soil weight behind a retaining wall

## RE: calculation of soil weight behind a retaining wall

Cedergren does caution - that with filters - failures have occurred but these were caused by materials used out of spec. (a sand and gravel having too much silt and clay size).

As for you last point - remember that all filter drains must have a positive outlet - in other words water entering must exit - same as dam drains.

## RE: calculation of soil weight behind a retaining wall

## RE: calculation of soil weight behind a retaining wall

You are talking about resistance. You are not talking about the applied horizontal force (i.e., active or at-rest earth pressures). You are also talking only about the sliding mode of failure. So, that's an, "Ntan(delta)" topic. The, "N" is based on the total or buoyant unit weight above the sliding surface. That total weight includes the soil above the failure surface and it includes the concrete above the failure surface. Both the soil and the concrete need to consider the position of the phreatic surface with buoyant below and total unit weights above.

Now the question is how to determine, "Delta," which is the interface friction angle. It may or may not be the same as phi.

Hope I helped!

f-d

ípapß gordo ainÆt no madre flaca!

## RE: calculation of soil weight behind a retaining wall

The bottom of the footing feels the effective stress of the soil and the upward pressure of the water. Frictional sliding resistance is equal to the effective stress normal to the bearing surface times the coefficient of friction between the concrete and the soil (this is smaller than the phi angle) times the area. If you take the total weight of the overlying soil, plus the total weight of the footing and wall, minus the hydrostatic uplift (pressure times area) you have the force that can be multiplied times the coefficient of friction to get the ultimate frictional sliding resistance.

You could definitely use the buoyant weight of the overlying soil and the buoyant weight of the footing as a shortcut and forget the hydrostatic uplift. But the shortcut makes is harder to keep track of the details. If the ground water at the supported side is at the ground surface, the water under the toe of the footing probably does not have the full hydrostatic head, or it would be squirting out from under the footing. So the hydrostatic uplift pressure must not be uniform. If a toe drain is provided to dispose of the seepage while preventing piping, the hydrostatic uplift pressure may be a triangle. It is important to know that when you calculate overturning moments.